Electrophotographic process in which the photoreceptor is repeatedly used has been applied to various business and office machines such as copying machines, output printers for computers and word processors because it is possible to use plain paper and obtain high density clear images.
As the photoreceptors used in such an electrophotographic process, inorganic photoconductors such as Se, CdS have been used. Recently, organic photoconductors have been developed and widely used.
On the other hand, with respect to the structures of the photoreceptors, a laminated photoreceptor which has an electroconductive support on which a charge-generating layer and a charge-transporting layer are laminated in that order is interested in because of their excellent sensitivity, durability and productivity as well as no pollution.
In the electrophotographic process using the laminated photoreceptor, the photoreceptor should be negatively charged since the charge-transporting layer in the photoreceptor usually comprises a hole transport substance. The electrophotographic process using the negatively charged photoreceptor is disadvantageous as compared with the positively charged electrophotographic process. Because, it is necessary in the former process to use a negative corona charger which accompanies the production of much ozone and to use a positive toner with poor durability. Thus, the positively charged electrophotographic process with no use of the laminated photoreceptor is desired.
Further, it is difficult and troublesome to prepare the laminated photoreceptor because the charge-generating layer should be formed with a thin and uniform thickness and a large area and the combinations of raw materials of which both layers consist are limited. In this regard, the electrophotographic process with no use of the laminated photoreceptor is strongly desired.
As the photoreceptor other than the laminated photoreceptor, a photoreceptor comprising a photosensitive layer which is formed by dispersing a particulate charge-generating substance in a binder containing a charge-transporting substance and a binder resin has been known. In the use of this dispersed photoreceptor, the above-mentioned problems concerning the use of the laminated photoreceptor can be resolved because it is possible to be positively charged and to prepare as the monolayer in principle. Especially, the dispersed photoreceptor containing the dispersed particles of the charge-generating substance in the relatively small amount was found to have the improved electric properties and little fatigue.
When the dispersed photoreceptor is repeatedly used in the electrophotographic process, however, it offers a problem such as change in charged voltage and lowering in sensitivity. Particularly when the dispersed photoreceptor is repeatedly used in the electrophorographic process including the means (or step) for optically erasing the residual charges on the photoreceptor after transfer, the above problem is important.
In the electrophotographic process using the laminated photoreceptor, as the light for erasing the residual charges the light which won't be absorbed in the charge-transporting layer is generally used. A light of relatively long wavelength such as tungsten lamp filtered to eliminate the shorter wavelength light and red light are often used. When such a light of relatively long wavelength is applied in the electrophotographic process using the laminated photoreceptor, the electrical fatigue such as the change of the sensitivity and the charged voltage of the photoreceptor can be minimized in its repeated use. However, if the same light is applied in the electrophotographic process using the dispersed photoreceptor, the decrease of the sensitivity and sometimes the raise of the charged voltage of the photoreceptor in its repeated use were observed.
The present inventors have investigated the stabilization of the properties of the dispersed photoreceptor in its repeated use and as the result, they discovered that the properties of the dispersed photoreceptor can be stabilized or kept when a light in the specified wavelength, which is strongly absorbed in the photosensitive layer and is small in distance of penetration into the photosensitive layer is used as the light for optically erasing the residual charges (hereinafter referred to as "charge erasing light") in the electrophotographic process.